A layout design system, semiconductor device using the layout design system, and fabricating method thereof are provided. The fabricating method of a semiconductor device includes loading a first layout, wherein the first layout comprises a first active region and a first dummy region, and the first active region comprises a fin-type pattern design having a first width, generating a second layout by substituting the fin-type pattern design with a nanowire structure design and forming a nanowire structure by using the second layout, wherein the second layout comprises a second active region in the same size as the first active region, and a second dummy region in the same size as the first dummy region, the nanowire structure design has a second width greater than the first width, and the nanowire structure comprises a first nanowire extending in a first direction, a second nanowire extending in the first direction and being formed on the first nanowire at a spacing apart from the first nanowire, a gate electrode surrounding a periphery of the first nanowire and extending in a second direction of intersecting with the first direction, a gate spacer being formed on a sidewall of the gate electrode and comprising an inner sidewall and an outer sidewall facing each other, the inner sidewall of the gate spacer facing a side surface of the gate electrode, and a source/drain epitaxial layer on at least one side of the gate electrode and being connected to the first nanowire.